Assessing the long‐term carbon‐sequestration potential of the semi‐natural salt marshes in the European Wadden Sea

Abstract

Salt marshes and other blue carbon ecosystems have been increasingly recognized for their carbon (C)-sink function. Yet, an improved assessment of organic carbon (OC) stocks and C-sequestration rates is still required to include blue C in C-crediting programs. Particularly, factors inducing variability in the permanence of sequestration and allochthonous contributions to soil OC stocks require an improved understanding. This study evaluates the potential for long-term C sequestration in the semi-natural salt marshes of the European Wadden Sea (WS), conducting deep (1.3 m) down-core OC-density assessments in sites with known site histories and accretion records. Because these young marshes have developed from tidal-flat ecosystems and have undergone rapid succession during the last 80-120 yr, the identification of different ecosystem stages down-core was crucial to interpret possible changes in OC density. This was conducted based on the down-core distribution of different foraminiferal taxa and grain sizes. Comparisons of historic and recent accretion rates were conducted to understand possible effects of accretion rate on down-core changes in OC density. d 13 C in OC was used to assess the origin of accumulated OC (autochthonous vs. allochthonous sources). We show that large amounts of short-term accumulated OC are lost down-core in the well-aerated marsh soils of the WS region and thus emphasize the importance of deep sampling to avoid overestimation of C sequestration. Despite steep declines in OC-density downcore, minimum values of OC density in the salt-marsh soils were considerably higher than those of the former tidal-flat sediments that the marshes were converted from, illustrating the greater C-sequestration potential of the vegetated ecosystem. However, our data also suggest that marine-derived allochthonous OC makes up a large fraction of the effectively, long-term preserved OC stock, whereas atmospheric CO 2 removal by marsh vegetation contributes relatively little. The implication of this finding for C-crediting approaches in blue C ecosystems has yet to be clarified.

Unrecognized controls on microbial functioning in Blue Carbon ecosystems: The role of mineral enzyme stabilization and allochthonous substrate supply

“…In our site, silt and clay contents represent approx. 80% of the deposited mineral matter and their contributions do not vary across the elevation gradient within the marsh (Butzeck et al, ; Mueller et al, ). In accordance with this, slopes and y ‐intercepts of the relationships between EEA per unit dry weight and organic/mineral content are relatively consistent across zones, indicating similarly active mineral‐associated enzyme pools (Figure a).…”

Origin of organic carbon in the topsoil of Wadden Sea salt marshes

“…Soils in both sites have OM contents <15% and are dominated by grain-sizes <63 µm (>70% silts and clays) (Mueller et al 2019). Flooding of the extensive high-marsh platforms, that this study is restricted to, only occurs during storm events predominantly in winter (Müller et al 2013a, Butzeck et al 2015.…”

No Detectable Broad-Scale Effect of Livestock Grazing on Soil Blue-Carbon Stock in Salt Marshes

“…Environmental context determines much of the external carbon inputs to salt marshes. For instance, sedimentation rates are linked to the local tidal regime (Stumpf, 1983) and allochthonous organic matter, that imported with marine sediment rather than created on the salt marsh, can contribute considerably to SOC stocks where tall vegetation traps sediment (Van de Broek et al, 2018;Mueller et al, 2019). Arguably, the impact of grazing on saltmarsh carbon is comparatively weak relative to the influence of sharp background environmental gradients (Grime, 1974;Nolte et al, 2013).…”

“…Salt marshes, along with mangroves and seagrass beds, are thought to provide longer-lasting and far denser "blue" carbon stores than most terrestrial systems, although salt marsh carbon stocks do vary considerably with marsh maturity, geomorphology and environmental setting (Sousa et al, 2010;Mcleud et al, 2011;Hayes et al, 2017;Himes-Cornell et al, 2018). Carbon stores are rich and long-lasting in marshes because they have high plant productivity (Middelburg et al, 1997) and sulfate rich, anaerobic sediments with slow organic deposition rates (Howarth, 1984;Valiela et al, 1985;Mueller et al, 2017) The low-energy, depositional characteristics of salt marshes also make them excellent carbon sinks (Chmura et al, 2003;Andersen et al, 2010) as much of the particulate material trapped by marshes is rich in externally produced organic matter (Van de Broek et al, 2018;Mueller et al, 2019).…”